1. Field of the Invention
The present invention generally relates to mobile communication systems.
Detailed descriptions of mobile communication systems can be found in the literature, in particular in Technical Specifications published by standardisation bodies such as in particular 3GPP (3rd Generation Partnership Project).
2. Description of the Prior Art
In such systems, a mobile terminal or User Equipment (UE) has access to mobile services offered by a Core Network (CN) via an Access Network (AN), such as in particular a Radio Access Network (RAN).
There are different types of mobile services such as in particular PS-based services (where PS stands for Packet Switched) and IP-based services (where IP stands for Internet Protocol). The Core Network CN therefore comprises different domains and/or subsystems such as in particular PS domain and operator's IP network. In particular UE has access to IP based services offered by an operator's IP network via an Access Network AN comprising: Radio Access Network RAN, and Core Network PS domain providing IP connectivity.
Existing or Legacy systems include in particular 3G (3rd Generation) systems. A typical example is UMTS (Universal Mobile Telecommunications System), including RAN called UTRAN (UMTS Terrestrial Radio Access Network), and GPRS Packet Core Network (where GPRS stands for General Packet Radio Service).
Now there is an evolution towards next generation or evolved systems, having enhanced performances. A typical example is 3G LTE (Long Term Evolution), including RAN called Evolved RAN, and CN called Evolved Packet Core Network.
Mobility Management (MM) is an important issue in such systems. There are different MM functions such as in particular location update functions and paging functions, the aim of such functions being to guarantee that a User Equipment UE can be reached even when there is no active radio connection between the UE and the network.
In Legacy systems as well as in Evolved systems, MM functions are performed at CN level, between UE and a CN-level MM Entity (MME), such as Legacy MME (for example Serving GPRS Support Node SGSN) in 3G Legacy systems, or LTE MME in 3G LTE systems. For PS connections, different MM states have been defined including in particular the following PMM states as defined for Legacy 2.5G GPRS systems for example (where PMM stands for Packet Mobility Management). In PMM Connected state (where a packet signalling connection is established between UE and CN), CN knows the location of the UE with the accuracy of a cell. In PMM Idle state (where no packet signalling connection is established between UE and CN), CN knows the location of the UE with the accuracy of a Routing Area (RA). UE performs a RA update when its RA changes. UE may move from PMM Connected state to PMM Idle state if there is not enough activity on the packet connection. If there is again some activity on the downlink for UE in PMM Idle state, paging of the UE is required to re-activate the packet connection. Paging is performed over the last Routing Area (RA) where the UE is known to be located, i.e. where it is registered.
In 3G Legacy systems such as in particular UMTS, CN is released from some MM functions which can be performed at RAN level, such as in particular at UTRAN level for UMTS (where UTRAN stands for UMTS Terrestrial Radio Access Network). For PS connections, different MM states have been defined including in particular the following states as defined for UTRAN-level MM for example. In a state called hereafter Radio Connected state, where Radio Bearers are established between UE and UTRAN, UTRAN knows the location of the UE with the accuracy of a cell. In a state called hereafter Non Radio Connected URA-PCH state (where URA stands for UTRAN Registration Area and PCH stands for Paging Channel), where no Radio Bearers are established between UE and UTRAN but a signaling connection or Signaling Radio Bearer is established between UE and UTRANUTRAN knows the location of the UE with the accuracy of URA. UE performs URA update when its URA changes. UE may move from a Radio Connected state to Non Radio Connected URAPCH state if there is not enough activity on this connection and user's mobility is such that cell updates would occur too frequently. If there is again some activity on the downlink to UE in URA-PCH state, paging is required to re-establish the radio connection. Paging is performed over the last URA where UE is known to be located, i.e. where it is registered. Non Radio Connected URA-PCH state, as seen from UTRAN, is seen from CN as PMM Connected state. In a mode called RRC Idle mode (where RRC stands for Radio Resource Control), no Radio Bearers and no Signaling Radio Bearers are established between UE and UTRAN. RRC Idle mode as seen from UTRAN is seen form CN as PMM Idle state.
The present invention more particularly relates to MM in a system architecture supporting mobility between various access systems such as in particular Legacy and Evolved access systems, enabling in particular to provide IP-based services through various IP connectivity access networks.
An example of system architecture supporting mobility between LTE and Legacy access systems is illustrated in FIG. 1.
The architecture illustrated in FIG. 1 comprises:                Multimode (Legacy/LTE) User Equipment (UE),        Legacy 3G Radio Access Network, such as for example UTRAN (UMTS Terrestrial Access Network), including Radio Network Controller (RNC) and Legacy Base Stations or Legacy Node B (such as in the illustrated example Legacy NodeB 1, Legacy NodeB 2, Legacy NodeB 3),        LTE Radio Access Network, such as for example E-UTRAN, including LTE Base Stations or LTE Node B (such as in the illustrated example LTE NodeB 4 and LTE NodeB 5),        Legacy Mobility Management Entity (Legacy MME),        LTE Mobility Management Entity (LTE MME),        a Gateway SAEGW providing interconnection between access system (Legacy and/or LTE) on one side, and an external IP network, not illustrated, on the other side.        
In this example of architecture, there is an as loose as possible coupling within the network infrastructure (both Core Network CN and Radio Access Network RAN) between LTE and Legacy technologies. There are separate radio entities and separate entities handling the Core Network-level mobility. The coupling between Legacy and LTE technology is only provided by:                a signaling communication channel between the LTE-MME entity handling the Core Network level mobility over LTE coverage and the legacy-MME entity handling the Core Network level mobility over legacy coverage        Gateway SAEGW.        
In Idle mode, Legacy MME (respectively LTE MME) knows the location of the UE with the accuracy of a Routing Area (RA) (respectively Tracking Area (TA)). An UE performs a RA (respectively TA) update when its RA (respectively TA) changes. Paging is performed over the last RA (respectively TA) where the UE has been registered.
The area on which a UE is paged is generally called Paging Area (PA). PA can be a portion of a RA (respectively TA) if sequential paging is used.
In an architecture supporting mobility between LTE and Legacy access systems, such as the one illustrated in FIG. 1 for example, a concept of Equivalent RA(s) or TA(s) (or Equivalent or Common Paging Area) can be used to limit the amount of signalling due to Idle mode mobility (in particular to minimize location update messages towards a subscriber database such as Home Location Register (HLR)). Equivalent Paging Area is made of a set of RA(s) and a set of TA(s). With this concept, an UE is simultaneously attached to each coverage i.e. “attached” (registered) on both legacy-MME and LTE-MME. When UE moves between LTE Idle state, and RRC Idle mode or URA PCH state, UE does no signaling to the network as long as the new cell is within one of the set of Equivalent RA(s) or TA(s). In the example of FIG. 1, TA1 and RA1 are in the same Equivalent Area.